Imaging Bolometry Development for Large Fusion Devices

Abstract

As magnetic fusion experiments progress towards longer time-scales, more intense neutron and gamma backgrounds, larger physical dimensions, and use of diagnostics for real-time control, the measurement of total radiated power (including total neutral particle fluxes) becomes more challenging. The use of thin foils that absorb and convert the radiation and particle fluxes into a measurable temperature rise, is a standard tool for the plasma diagnostician. Both wide-angle, and tightly collimated foil detector arrays have been used on nearly every tokamak or alternative concept device. The temperature readout is usually accomplished via a DC or AC bridge circuits, to recover the millivolt-level signals, and then differentiated to get power from the time-integrated heat load on the foil and substructures. The challenge lies in making fast bolometers, responsive over the expected range in incident energies, which are also robust, reliable, compact, convenient to calibrate, and cheap enough to afford hundreds of channels for good spatial resolution using tomographic techniques. So-called “ASDEX” or JET-style bolometers manufactured by PTS GmbH, represent the present standard of performance. A new imaging bolometer¹, using infrared readout of hundreds of mini-foils in an actively cooled mask, has been built by LANL, and is presently being tested on the CHS and LHD plasmas in Japan. We describe work in progress towards a radically different approach to obtaining bolometer signals, which has many advantages (and a few disadvantages) over present conventional bolometer systems. We explore a system using technology available today, which simultaneously allows for a cheaper per/channel cost, a high degree of radiationhardening, no wiring-harnesses in the vessel, no pre-amplifiers, good noise immunity, compatibility with long-pulse operation and real-time readout, and hundreds of channels to form a bolometric image. The drawbacks are the requirement for an optical path to the backside of the detector array, and some sacrifice of sensitivity. *